I was a fan of MSE until a few months ago when Mrs R received an attachment by email which she had saved, as it looked to her to be a genuine expected email, but panicked when instead of the pdf she was expecting on opening it, the Windows Run dialog box appeared.
To cut a long story short, after multiple AV installs, registry and file checking, it turned out that she hadn't gone as far as running the program, so it then came down to why MSE hadn't picked it up. The name and file size matched the genuine article exactly, which was first identified two months beforehand - a cryptolocker variant. Manually scanning it in MSE gave nothing but several online scanners picked it up straight away.
Would MSE have picked it up on running it? Was it a corrupted version that was not a threat? I don't know. However, Avast simply deletes these attachments from affected emails and puts Virus in the header - so far, I haven't seen anything remotely suspicious get through.